Plugin validator allows the user to specify unknown keys

[nataliecarey]

This allows the following syntax when running the validator:

npx govuk-prototype-kit@latest validate-plugin --keysToIgnoreIfUnknown="aProposedFeatures,anotherProposedFeature"

This means that people can add proposed features to their config before the interface is added to the validator as explained in https://github.com/alphagov/govuk-prototype-kit/issues/2312